Back to EveryPatent.com
| United States Patent |
5,762,999
|
|
Okawa
, et al.
|
June 9, 1998
|
Process for producing magnetic recording medium
Abstract
A process for producing a magnetic recording medium comprising applying a
magnetic coating composition to a substrate to form a magnetic layer and
subjecting the coated substrate to calendering, wherein the elastic roll
used in the calendering has a water absorption of 0.1 to 5% by weight per
unit volume and a Shore hardness of not less than 90, and the magnetic
layer immediately before being calendered has a residual solvent content
of 100 to 5000 ppm.
| Inventors:
|
Okawa; Masayuki (Tochigi-Ken, JP);
Sato; Masayasu (Tochigi-Ken, JP)
|
| Assignee:
|
Kao Corporation (Tokyo, JP)
|
| Appl. No.:
|
888895 |
| Filed:
|
July 7, 1997 |
| Current U.S. Class: |
427/130; 427/128; 427/365 |
| Intern'l Class: |
B05D 005/12 |
| Field of Search: |
427/128,130,365
|
References Cited
U.S. Patent Documents
| 4324177 | Apr., 1982 | Tsuji et al. | 100/155.
|
| Foreign Patent Documents |
| 51-92606 | Aug., 1976 | JP.
| |
| 58-194142 | Nov., 1983 | JP.
| |
| 1-94532 | Apr., 1989 | JP.
| |
| 4-149821 | May., 1992 | JP.
| |
Primary Examiner: Pianalto; Bernard
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. A process for producing a magnetic recording medium comprising applying
a magnetic coating composition to a substrate to form a magnetic layer and
subjecting the coated substrate to calendering, wherein the elastic roll
used in said calendering has a water absorption of 0.1 to 5% by weight per
unit volume and a Shore hardness of not less than 90, and the magnetic
layer immediately before being calendered has a residual solvent content
of 100 to 5000 ppm.
2. The process according to claim 1, wherein said elastic roll has an
elastic member comprising a polyester resin.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a magnetic
recording medium. More particularly, it relates to a process for producing
a magnetic recording medium having an improved output.
2. Description of Related Art
Aiming at obtaining a magnetic recording medium having an increased output,
a great number of attempts to achieve the aim from an aspect of production
process have been made to date. For example, to optimize various
calendering conditions, such as temperature, linear pressure, and material
of rolls, which have great influences on the surface properties of a
magnetic layer, namely the output of a magnetic recording medium, has been
studied. Attention has also been paid to other conditions in calendering
in an attempt to improve the output of a magnetic recording medium.
For example, Japanese Patent Laid-Open No. 94532/89 discloses a process for
producing a magnetic recording medium comprising applying a magnetic
coating composition to a nonmagnetic substrate to form a magnetic layer
and subjecting the coated substrate to calendering, in which a
polyurethane resin having a glass transition point of not lower than
30.degree. C. is used as a binder of the magnetic coating composition, and
the calendering is carried out at the time when the residual solvent
content of the magnetic layer is reduced to 0.1 to 3% by weight.
Japanese Patent Laid-Open No. 149821/92 teaches a process for producing a
magnetic recording medium in which the time required from charging of
magnetic powder in a mixer to the completion of calendering following
application of the magnetic coating composition is limited to a range of
from 2 to 48 hours.
Japanese Patent Laid-Open No. 194142/83 describes a process for producing a
magnetic recording medium comprising applying a magnetic coating
composition on a nonmagnetic substrate to form a magnetic layer and
subjecting the coated substrate to calendering, in which the humidity of
the environment for carrying out calendering is not lower than 60% RH.
Japanese Patent Laid-Open No. 92606/76 proposes to use a polyamide resin
having a Shore hardness of 70 or more as a elastic roll for polishing a
magnetic recording medium. The publication also refers to the water
content of the elastic roll, but the reference is made only in connection
with the frictional electrification. There is no mention of any influence
of the water content of the elastic roll upon mechanical deformation of
the roll in association with the Shore hardness.
While these conventional techniques have achieved an improvement in output
to some extent, it is still necessary to develop a magnetic recording
medium with a further increased output with which to meet the recent
demand for high-density recording.
SUMMARY OF THE INVENTION
An object of the present invention is t o provide a process for producing a
magnetic recording medium having an improved output.
The inventors of the present invention have conducted extensive
investigation to accomplish the above object. They have found as a result
that the output of a magnetic recording medium depends on a combination of
conditions of calendering following formation of a magnetic layer, i.e.,
water absorption and Shore hardness of an elastic roll used for
calendering and residual solvent content of the magnetic layer to be
calendered.
The present invention has been completed based on the above finding. The
above object of the present invention is accomplished by a process for
producing a magnetic recording medium comprising applying a magnetic
coating composition to a substrate to form a magnetic layer and subjecting
the coated substrate to calendering, wherein the elastic roll used in the
calendering has a water absorption of 0.1 to 5% by weight per unit volume
and a Shore hardness of not less than 90, and the magnetic layer
immediately before being calendered has a residual solvent content of 100
to 5000 ppm.
Material of a roll is one of important conditions of calendering as stated
above. The inventors have found for the first time that one of the
important factors influential on material quality of an elastic roll
consists in water absorption. Specifically, recent improvements in
material of an elastic roll have led to increased hardness of the roll.
The purpose of the improvements is to reduce deformation of an elastic
roll under pressure thereby controlling the contact area with a coating
film and obtaining a large pressing force. It has been found that the
water absorption of an elastic roll has an influence on the elastic force
of the roll (the mechanism of mechanical deformation). If an elastic roll
reduces its elastic force, it cannot fully exhibit its shearing effect,
i.e., calendering performance. In the present invention, improved
calendering performance can be obtained by controlling the water
absorption of the elastic roll within the above-specified range.
According to the process of the present invention, a magnetic recording
medium having a further improved output can be produced by performing
calendering under the above-mentioned conditions.
DETAILED DESCRIPTION OF THE INVENTION
The process for producing a magnetic recording medium according to the
present invention is hereinafter described in detail.
The present invention relates to a process for producing a magnetic
recording medium comprising applying a magnetic coating composition on a
substrate to form a magnetic layer, followed by calendering, i.e., a
process for producing a coated type magnetic recording medium. The process
of the present invention has its characteristics in the step of
calendering.
The characteristics reside in that:
(1) the elastic roll used in the calendering has a water absorption of 0.1
to 5% by weight per unit volume and a Shore hardness of not less than 90,
and
(2) the magnetic layer immediately before being calendered has a residual
solvent content of 100 to 5000 ppm.
These characteristics will be explained in order.
In the process of the present invention, the elastic roll used in
calendering has a water absorption of 0.1 to 5% by weight per unit volume.
The term "water absorption per unit volume ›W (%)!" as used herein means a
percentage change in weight of an elastic roll (only the elastic member of
a roll) due to water absorption, i.e., a percentage of a difference
between the weight of a roll after water absorption (W.sub.wet) and that
before water absorption (W.sub.dry) to the weight of the roll before water
absorption (W.sub.dry) as expressed by equation (1):
W(%)=›(W.sub.wet -W.sub.dry)/W.sub.dry !.times.100 (1)
In equation (1), W.sub.wet is the weight as measured after an elastic roll
has been soaked in water at 25.degree. C. for 20 days.
If the water absorption of an elastic roll is less than 0.1% by weight,
static electricity is generated as a substrate having formed thereon a
magnetic layer passes through calendering rolls, and the substrate is
damaged by a discharge between the substrate and the rolls. If the water
absorption exceeds 5% by weight, the elastic roll has a reduced elastic
force on its surface so that a calendering roll shows insufficient
calendering performance. A preferred water absorption of the elastic roll
is 0.1 to 0.5% by weight.
The elastic roll should have a Shore hardness of not less than 90. If the
Shore hardness is less than 90, the calendering effect is reduced in
association with the above-described water absorption, failing to obtain a
desired degree of smoothness. The Shore hardness of the elastic roll is
preferably 91 or more, still preferably 92 or more. The terminology "Shore
hardness" as used herein is a D value as measured according to JIS K-6301.
In the present invention, any elastic roll commonly used in the manufacture
of magnetic recording media can be used with no particular limitation as
far as the above-mentioned requirements of water absorption and Shore
hardness are fulfilled. Commonly employed elastic rolls are generally
composed of a metallic core covered with an elastic member. The elastic
member preferably has a thickness of 1 to 20 mm, particularly 5 to 15 mm,
for the following reasons. If the thickness of the elastic member is less
than 1 mm, the influence of the metallic core will be reflected on the
elastic member. On the other hand, an elastic member having a thickness
exceeding 20 mm is liable to breakage. Useful elastic rolls include a
cotton roll, a plastic roll, and a rubber roll. A plastic roll is
preferred. Above all, an elastic roll using a polyester resin as an
elastic member is particularly preferred for its modulus of elasticity.
While not limiting, the diameter of the elastic roll is preferably 15 to 35
cm, particularly 20 to 30 cm, for securing homogeneity of the roll.
Other conditions of calendering, e.g., the temperature of a metallic roll
and the linear pressure, are not particularly limited and can be decided
appropriately according to the kind and thickness of the substrate, the
kind of the magnetic coating composition, and the like. In general, the
temperature of a metallic roll is preferably 70 to 120.degree. C.,
particularly 80 to 100.degree. C., and the linear pressure is preferably
100 to 400 kg/cm, particularly 200 to 350 kg/cm.
Conventionally known calendering equipment, such as those described in
Japanese Patent Publication No. 2485/86, Japanese Patent Laid-Open No.
117335/81, and Japanese Patent Publication No. 4970/82, can be used in the
present invention with no particular limitation. In carrying out
calendering, a metallic roll is usually set on the magnetic layer side of
a magnetic recording medium, and the elastic roll on the other side, i.e.,
the substrate side.
The residual solvent content of the magnetic layer before being subjected
to calendering (i.e., after formation of the magnetic layer and
immediately before being calendered) will be explained below.
The residual solvent content of the magnetic layer before being subjected
to calendering ranges from 100 to 5000 ppm. When the condition that the
residual solvent content be within the range of from 100 to 5000 ppm is
satisfied, with the conditions of water absorption and Shore hardness
being also satisfied within the above-specified range, there are exhibited
extremely high calendering properties. A preferred residual solvent
content is 1000 to 3000 ppm, particularly 1500 to 2500 pm. The residual
solvent content as referred to herein is one calculated per unit weight of
the magnetic layer before being calendered, which can be measured by, for
example, gas chromatography.
The method for adjusting the residual solvent content of the magnetic layer
before calendering within the above-specified range is not particularly
limited. For example, the solvent of the coating film on the substrate is
evaporated by means of hot air drying, infrared drying, etc. before the
coated substrate is subjected to calendering to adjust the residual
solvent content within the above-specified range.
While calendering reduces the surface roughness of the magnetic layer, it
is preferable that the magnetic layer to be calendered has a surface
roughness of not more than about 40 nm, particularly 4 to 15 nm,
especially 4 to 12 nm. The term "surface roughness" as used herein means a
center-line average roughness at a cut off of 0.25 mm as specified in JIS
B-0601.
Any known magnetic or nonmagnetic substrate can be used in the present
invention with no particular limitation. Examples of useful substrates
include those comprising known resins, such as polyethylene terephthalate,
polyethylene naphthalate, polyphenylene sulfide, polycarbonate, polyamide,
polyimide, polyamidoimide, polysulfone, aramid, and aromatic polyamide;
those comprising a metal, such as aluminum and copper; and paper. If
desired, the substrate can be subjected to a surface treatment, such as a
corona discharge treatment, a plasma treatment, a treatment for
facilitating adhesion, a heat treatment, and a dust removal treatment,
before formation of a magnetic layer. A preferred thickness of the
substrate is usually 1 to 300 .mu.m.
The magnetic coating composition to be applied to the substrate generally
comprises magnetic powder, a binder, and a solvent and, if desired,
additives, such as a dispersant, a lubricant (e.g., fatty acids or esters
thereof), abrasives (e.g., alumina), an antistatic agent (e.g., carbon
black), and a hardening agent (e.g., polyisocyanate).
The magnetic powder includes ferromagnetic iron oxide-based magnetic
powder, such as FeO.sub.x (1.33<.times.<1.5) doped with Cr, M, Co, Ni or a
like metal; ferromagnetic chromium dioxide-based magnetic powder, such as
CrO.sub.2 and CrO.sub.2 doped with a metal or a metal oxide (e.g., Na, K,
Fe, Mn or an oxide thereof) or a non-metal (e.g., P); and ferromagnetic
metal powder, such as one having a metal content of at least 70% by
weight, with at least 80% by weight of the metal content comprising at
least one ferromagnetic metal (e.g., Fe, Co or Ni).
Illustrative examples of suitable binders for use in the invention include
cellulose resins, vinyl chloride-vinyl acetate copolymers, vinyl
chloride-vinylidene chloride copolymers, vinyl chloride-vinyl
acetate-vinyl alcohol copolymers, epoxy resins, and polyurethane.
Commercially available products, such as MR110 (epoxy-containing vinyl
chloride copolymer produced by Nippon Zeon Co., Ltd.), VAGH (produced by
Nagase Sangyo K.K.), and UR8200 (polyurethane produced by Toyobo Co.,
Ltd.), may be used as a binder.
Any solvent can be used with no limitation as long as is suitable for
dissolving the binder. Examples of suitable solvents include ketones, such
as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; esters,
such as ethyl acetate; ethers, such as tetrahydrofuran and dioxane;
aromatic hydrocarbons, such as benzene and toluene; and chlorinated
hydrocarbons.
The magnetic coating composition obtained by mixing and dispersing the
above components can be applied to a substrate by a known coating
technique, such as gravure coating, spray coating or roll coating. The
magnetic layer thus formed preferably has a dry thickness of 0.1 to 5
.mu.m, particularly 0.2 to 3 .mu.m.
The process of the present invention is applicable to both a so-called
in-line calendering system, in which the steps of from applying a magnetic
coating composition to a substrate through calendering are conducted in a
continuous manner, and a so-called off-line calendering system, in which a
substrate having formed thereon a magnetic layer is once taken up in roll
and then subjected to calendering.
In the process of the present invention, various steps other than those
described above that are generally taken in the manufacture of magnetic
recording media can be used with no particular limitation. For example,
the coating film formed by applying the magnetic coating composition can
be subjected to a treatment in a magnetic field for orientating the
magnetic powder of the film before the film is dried. The magnetic layer
may be subjected to finishing, such as polishing or cleaning. The magnetic
coating composition may be applied by a well-known successive coating
method. In the production of a magnetic tape as a magnetic recording
medium, the coated film after calendering is aged at a prescribed
temperature for a prescribed time, followed by slitting to a desired width
.
The present invention will now be illustrated in greater detail with
reference to Examples, but it should be understood that the present
invention is not construed as being limited thereto. Unless otherwise
noted, all the parts are given by weight.
EXAMPLE 1
A magnetic tape was prepared as follows.
The following components were mixed and dispersed and diluted to prepare a
magnetic coating composition.
______________________________________
Magnetic Coating Composition:
______________________________________
Acicular magnetic metal powder mainly
100 parts
comprising iron (coercive force: 1500 Oe;
saturation magnetization: 130 emu/g;
average major axis length: 0.18 .mu.m)
Alumina (average particle size: 0.1 .mu.m)
12 parts
Carbon black (average primary particle
1 part
size: 20 nm)
MR110 (vinyl chloride copolymer produced
12 parts
by Nippon Zeon Co., Ltd.)
UR8200 (polyurethane produced by Toyobo
8 parts
Co., Ltd.)
2-Ethylhexyl stearate (lubricant)
1 part
Palmitic acid (lubricant)
2 parts
Coronate L (hardening agent produced by Nippon
3 parts
Polyurethane Industry Co., Ltd.)
Methyl ethyl ketone (solvent)
120 parts
Toluene (solvent) 80 parts
Cyclohexanone (solvent) 40 parts
______________________________________
The resulting coating composition was applied to a polyethylene
terephthalate (PET) substrate to a dry thickness of 2.3 .mu.m. While the
coating film was wet, the coated substrate was passed through a solenoid
type magnet of 8000 Oe to have the magnetic powder orientated, dried in
hot air at 80.degree. C., and rolled up. The coated substrate was then
calendered by means of calendering equipment composed of a metallic roll
and an elastic roll (diameter: 25 cm) comprising a polyester resin under
conditions of a linear pressure of 300 kg/cm, a metallic roll temperature
of 100.degree. C., and a speed of 100 m/min. The metallic roll was set on
the magnetic layer side, and the elastic roll on the opposite side. After
aging, the coated substrate was slit to a prescribed width to obtain a
magnetic tape.
The gloss and output of the resulting magnetic tape were measured. The
results obtained are shown in Table 1 below along with the production
conditions (the water content and the Shore hardness of the elastic roll
and the residual solvent content of the magnetic layer).
EXAMPLES 2 TO 7 AND COMPARATIVE EXAMPLES 1 TO 9
Magnetic tapes were produced using the same substrate and the same magnetic
coating composition as used in Example 1 under the conditions shown in
Table 1. The same measurements as in Example 1 were made on the resulting
magnetic tapes. The results are shown in Table 1.
TABLE 1
______________________________________
Water Residual
Absorp- Solvent Roll Material &
Output
tion Content Shore (8 MHz)
Example No.
(wt. %) (ppm) Hardness Gloss
(dB)
______________________________________
Example 1
1.0 1810 Polyester 210 +2.8
Example 2
0.5 1700 61 214 +3.0
Example 3
4.4 2600 187 +1.2
Example 4
4.0 1800 193 +1.6
Example 5
3.1 3400 195 +1.9
Example 6
1.0 190 192 +1.5
Example 7
0.4 301 Polyester 208 +3.0
93
Compara.
1.2 80 Polyester 170 -0.2
Example 1 93
Compara.
1.2 6110 166 -0.5
Example 2
Compara.
1.2 5050 168 -0.2
Example 3
Compara.
5.2 1700 Polyester 170 +0.0
Example 4 91
Compara.
5.8 1800 160 -0.5
Example 5
Compara.
0.07 301 209 --*
Example 6
Compara.
0.8 2200 Nylon 172 +0.1
Example 7 80
Compara.
1.3 2430 171 +0.1
Example 8
Compara.
2.4 2100 169 0
Example 9
______________________________________
Note:
*Suffering from a coating film defect due to static electrification.
As is apparent from the results in Table 1, the magnetic tapes that are
produced under controlled calendering conditions of water absorption and
Shore hardness of the elastic roll and residual solvent content of the
magnetic layer (Examples 1 to 7) exhibit a high gloss and a high output.
On the other hand, in Comparative Examples 4 to 6 where the residual
solvent content of the magnetic layer and the Shore hardness of the
elastic roll are within the respective ranges specified in the present
invention but the elastic roll has a high water absorption, both the gloss
and the output of the magnetic tapes are on low levels.
In Comparative Examples 1 to 3 and 7 to 9 wherein the water content of the
elastic roll is within the range specified in the present invention but
the residual solvent content and the Shore hardness are out of the range
of the present invention, both the gloss and the output of the magnetic
tapes are on low levels.
Top